A device which is currently available is characterised by a capsule comprising a hollow tubular body having an opening at one end, a plunger driven by a spring for urging a solid therapeutic composition contained in the body towards the opening, an annular flange around the opening to prevent the driving means for expelling the solid composition from the body, and two resilient arms which are attached to a cap which seals the tubular body of the other end.
A difficulty with the present device is that the cap is welded onto the hollow body. The welding is often not efficient and allows entry of water and ruminal fluid into the capsule. The therapeutic composition is usually a hygroscopic composition and in the presence of water swells and eventually ruptures the capsule. Poor welding can also result in capsule rupture which is not desirable because the sudden release of the whole of the therapeutic composition intended for sustained release can cause death of the animal.
Another difficulty is the inclusion of vent holes in the cap. The vent holes are needed so that a vacuum will not form when the capsule is administered. Prior to and during administration of the capsule, the lubricant has a tendency to leak through the vent holes. Loss of lubricant from the vent holes is not desirable. The lubricant acts as a seal and provides a barrier film. Any loss of lubricant could result in erratic payout of the therapeutic composition.
Another difficulty with the present device is that the tlanged edge of the piston provides a poor seal. The poor sealing results in water entering the capsule thereby causing friction as the piston moves along the tube. This friction is responsible for erratic payout of the therapeutic composition. Furthermore piston failure will result in water entering the capsule to cause swelling of the therapeutic composition and increased friction.
There is need for a sustained release capsule which can effectively remain sealed throughout the retention time in the rumen. There is also a need for a sustained release capsule which can continuously deliver a therapeutic or growth stimulating composition to range-fed ruminants, such as cattle or sheep.